Electricity

Electrostatics and Fields

Electric Charges

 electricity comes from the Greek word elektron, which means amber.

 Amber is petrified tree resin and the Greeks noticed that if you rubbed an amber rod with a cloth it would attract small bits of leaves  Today rubbing a balloon, a rubber rod, a glass rod or any number of substances causes attraction of other objects (Charging an object)

 There are only two possible types of charge.

 Positive charge – rubbing glass rod with silk  Negative charge – rubbing rubber rod with fur  Unlike charges attract and like charges repel  Charged objects also tend to attract neutral objects

Repulsion/Attraction of Charges

What Happens when a object becomes charged?

    Atoms have three subatomic particles: protons (+), electrons (-) and neutrons In neutral atoms the # of protons = The # of electrons The electrons, with addition of some energy, can be removed and transferred to other objects Rod + fur – electrons move from fur to rod

Charging Objects

Equal number Of electrons And protons Rubbing provides energy which Helps to remove electrons from The cloth

 Electrons move,

but protons do not

! Positively charged objects usually end up that way because they have

lost

electrons leaving them with more protons that electrons  Remember – we are just moving electrical charge from place to place

Conduction and Induction

 There are other ways to give an object an electrical charge  Insulators vrs conductors Charged/neutral Conductor (gold, copper) Allows electrons to move Through it Insulator (rubber, glass) Does not allow electrons To move through it

Conduction

  Bring a neutral object in contact with a previously charged object. Both objects will end up with the same charge although it will be smaller than the initial charge Electroscope – used to illustrate the behaviour of electrical charges Metal knob Container Minimizes air resistance Gold leaves

Conduction

Electrons move up into the rod, Leaving the protons behind to Repel each other Electrons move into the electroscope And move down the leaves causing Them to repel

Induction

 A charged object is just brought near a neutral object. The charged object never actually touches the neutral object.

The electrons flow to the top to be close To the positive rod leaving the protons Behind to repel each other

Induction

 The electrons flow downward away from the neg. rod and they repel each other

Using Induction to Determine Charge

Start with an electroscope with neg. charge e- move up (attracted To pos. charge) and the leaves move closer together e- move down (away from the neg. rod) the leaves move farther apart

Grounding

 The Earth because of its immense mass can gain or lose many negative (or positive) charges and still remain neutral.

 As a result when you make contact between a charged object and the Earth it will immediately lose its charge and become neutral.  This process is known as grounding. The third wire on an electrical outlet is grounded so as to avoid any dangerous charge buildup

Grounding

When grounded an electroscope will lose or gain electrons until it becomes neutral

Electrical Forces

 We have seen how electrically charged objects can repel or attract other charged objects.  Is there a way to mathematically describe the attraction/repulsion? This question was answered by Charles Coulomb .  the greater the quantity of charge involved the greater the force of attraction or repulsion. Also, The force of attraction/repulsion actually depends on the

square

of the distance.

 F = force of attraction/repulsion (Newtons or N)  q 1 and q 2 = quantity of charge on each object (coulombs or C)  d = distance between the two charged objects (m)  K = 9.00x10

9 N m 2 /C 2

Conversions

 Typically charges are usually measured in micro-coulombs ( μC)  ( 1C is a large quantity of charge)  Is there a limit to how small a charge can be? As it turns out there appears to be. No charge smaller than the charge found on an electron (or proton) seems to exist. This quantity of charge is called the elementary charge and has a value of:  e= 1.60 x 10 -19 C

Examples

 Two positive charges each of quantity10 µC are separated by a distance of 5.00x10-8 m. What force will each charge experience?

Three examples

 A 30 µC charge is placed 40 cm from a second unknown charge. If the 30 µC charge experiences a net attractive force of 25.3 N, what is value of the second charge?  A 40 µC and -20 µC charge exert a force of magnitude 1000 N on each other. How far apart are the two charges?  How many protons are needed to have 1.0 C of charge?

Example

 Three charges are arranged at the corners of a right triangle as shown below. Calculate the net force exerted on charge B.

 Both charges A and C exert a force on B. The net force on B will therefore be the resultant of the force that A and C exert on it. Even though A and C exert a force on each other these forces have no effect on B so we can ignore them